There are several types of microphones and related transducers, such as for example, dynamic, crystal, condenser/capacitor (externally biased and electret), etc., which can be designed with various polar response patterns (cardioid, supercardioid, omnidirectional, etc.) All of these types have their advantages and disadvantages depending on the application. Condenser microphones are able to respond to very high audio frequencies, and they are usually much more sensitive than dynamic microphones, making them more suitable for quieter or distant sound sources. Such frequency responses are possible because the diaphragms of condenser microphone transducers can typically be made thinner and lighter than those of dynamic models due to the fact that, unlike dynamic models, the diaphragms do not have the mass of a voice coil attached thereto within the acoustical space of the transducer. On the other hand, one of the advantages of dynamic microphones is that they are passive and therefore do not require active circuitry to operate. As such, dynamic microphones are generally robust, relatively inexpensive, and less prone to moisture/humidity problems. They also exhibit a potentially high gain before feedback becomes a problem. These attributes make them ideal for on-stage use.
A phenomenon that all directional microphone transducer designs must contend with is called the “proximity effect.” The proximity effect is an increase in low frequency (bass) response when the microphone is used close to the sound source. This increased response is caused by the fact that directional microphones also capture sound waves from the rear of the transducer capsule, which is delayed in an acoustic passage or port and then added to the sound energy arriving on-axis. When the sound source is relatively distant, the phase shift introduced by the acoustic passage causes sound waves arriving from the rear to primarily be cancelled out when substantially the same sound levels arrive at the front and rear of the microphone transducer. For relatively close sound sources, however, the inverse square law dictates that there will be an increased sound level at the front of the microphone transducer than the sound level at the rear. This reduces the efficiency of the port in cancelling low frequencies. Pragmatically speaking, a vocalist, speaker, musical instrument or other sound source that is positioned close to the microphone will produce a significant amount of bass response.
The typical strategy for dealing with the proximity effect is to reduce low frequency output (high pass) either electrically or mechanically through increased mechanical resonance. One mechanical strategy employs an additional compliance element, such as a second diaphragm, which can be placed in series with the rear port tuning impedance to control the proximity effect. Such dual diaphragm microphone transducers, however, have been limited to condenser-type microphone applications because of the smaller size and simplicity of the acoustical space within condenser microphone transducers.
There is a need for a dual diaphragm dynamic type microphone transducer that, among other things, provides control of source/receiver proximity effects without sacrificing professional level dynamic microphone performance.